In recent years, various techniques for crystallizing or improving the crystallinity of an amorphous or polycrystalline semiconductor film have been investigated. This technology is used in the manufacture of a variety of devices, such as image sensors and active-matrix liquid-crystal display (AMLCD) devices. In the latter, a regular array of thin-film transistors (TFT) are fabricated on an appropriate transparent substrate, and each transistor serves as a pixel controller.
Semiconductor films are processed using excimer laser annealing (ELA), also known as line beam ELA, in which a region of the film is irradiated by an excimer laser to partially melt the film and then is crystallized. The process typically uses a long, narrow beam shape that is continuously advanced over the substrate surface, so that the beam can potentially irradiate the entire semiconductor thin film in a single scan across the surface. ELA produces small-grained polycrystalline films; however, the method often suffers from microstructural non-uniformities which can be caused by pulse to pulse energy density fluctuations and/or non-uniform beam intensity profiles.
Sequential lateral solidification (SLS) using an excimer laser is one method that has been used to form high quality polycrystalline films having large and uniform grains. A large-grained polycrystalline film can exhibit enhanced switching characteristics because the reduced number of grain boundaries in the direction of electron flow provides higher electron mobility. SLS processing also provides controlled grain boundary location. U.S. Pat. Nos. 6,322,625 and 6,368,945 issued to Dr. James Im, and U.S. patent application Ser. Nos. 09/390,535 and 09/390,537, the entire disclosures of which are incorporated herein by reference, and which are assigned to the common assignee of the present application, describe such SLS systems and processes.
In an SLS process, an initially amorphous (or small grain polycrystalline) silicon film is irradiated by a very narrow laser beamlet. The beamlet is formed by passing a laser beam pulse through a slotted mask, which is projected onto the surface of the silicon film. The beamlet melts the amorphous silicon and, upon cooling, the amorphous silicon film recrystallizes to form one or more crystals. The crystals grow primarily inward from edges of the irradiated area toward the center. After an initial beamlet has crystallized a portion of the amorphous silicon, a second beamlet is directed at the silicon film at a location less than the lateral growth length from the previous beamlet. Translating a small amount at a time, followed by irradiating the silicon film, promotes crystal grains to grow laterally from the crystal seeds of the polycrystalline silicon material formed in the previous step. As a result of this lateral growth, the crystals are of high quality along the direction of the advancing beamlet. The elongated crystal grains are separated by grain boundaries that run approximately parallel to the long grain axes, which are generally perpendicular to the length of the narrow beamlet. See FIG. 6 for an example of crystals grown according to this method.
When polycrystalline material is used to fabricate electronic devices, the total resistance to carrier transport is affected by the combination of barriers that a carrier has to cross as it travels under the influence of a given potential. Due to the additional number of grain boundaries that are crossed when the carrier travels in a direction perpendicular to the long grain axes of the polycrystalline material or when a carrier travels across a larger number of small grains, the carrier will experience higher resistance as compared to the carrier traveling parallel to long grain axes. Therefore, the performance of devices fabricated on polycrystalline films formed using SLS, such as TFTs, will depend upon the crystalline quality and crystalline orientation of the TFT channel relative to the long grain axes, which corresponds to the main growth direction.
Devices that utilize a polycrystalline thin film often do not require that the entire thin film have the same system performance and/or mobility orientation. For example, the pixel controller regions of a AMLCD device may require certain mobility performance and orientation, while the mobility requirements for the column and row drivers (the integration regions) may be considerably greater than for the pixel controllers region and also may be oriented differently. Processing the entire film surface, e.g., the integration regions and the pixel controller regions, under the conditions necessary to provide the high mobility requirements of the integration regions of the TFT can be inefficient and uneconomical since excess irradiation and processing time of the lower performance regions of the thin film may have been expended with no gain in system performance. To achieve acceptable system performance while optimizing manufacturing processing time and/or minimizing the energy expended in irradiating a thin film sample, laser beam pulses having different energy beam characteristics e.g., beam energy profile (density), beam shape, beam orientation, beam pulse duration, etc, can be used to process different regions of the thin film sample. This can be accomplished with systems that utilize a single optical path.
In systems having a single optical path, one or more of the optical elements and the mask (if present) can be adjusted, inserted or substituted, etc., within the optical path so as provide a laser beam pulses having different energy beam characteristics. Additionally, the orientation of the substrate, relative to the orientation of the incoming laser beam pulses, can also be adjusted to effectively produce a laser beam pulse that has different energy beam characteristics. For example, the system can include a mask that is rotatable via a mask holder. The mask is held in a first position to facilitate the irradiation processing of a first portion of the silicon film and then is rotated to a second position, e.g., rotate 90°, to facilitate the irradiation processing of a second portion of the silicon film. The system can include two masks having different masking shapes being located on a mask holder. To irradiate a first portion of the silicon film, the first mask is aligned with the laser beam optical path via the mask holder. To irradiate a second portion, the second mask is then aligned with the laser beam optical path via the mask holder, e.g., the mask holder can be a rotatable disk cartridge. Thus, laser beam pulses having different energy beam characteristics can be generated and delivered to the amorphous silicon film on the same optical path in this manner. Other prior art systems include an adjustable demagnification optical element. To generate laser beam pulses having differing energy beam characteristics, the adjustable demagnification optical element is set to a first magnification during the irradiation of a portion of the amorphous silicon film and then set to a different magnification during the irradiation of another portion of the amorphous silicon film.
Generating laser beam pulses with different energy beam characteristics along a single optical path can cause the crystallization processing times to otherwise increase since the delivery of the irradiation energy to the amorphous silicon film may need to be interrupted to facilitate the modulation of the energy beam characteristics. In this instance, a system having a single optical path may not be advantageous since the changing of the optical elements, the mask configuration or orientation, or the substrate orientation, etc., to facilitate an adjustment of the laser beam pulse's energy beam characteristics could dramatically lower the duty cycle of the delivered laser energy.